FORGED TiAl COMPONENTS, AND METHOD FOR PRODUCING SAME

ABSTRACT

The present invention relates to a method for producing forged components of a TiAl alloy, in particular turbine blades, wherein the components are forged and undergo a two-stage heat treatment after the forging process, the first stage of the heat treatment comprising a recrystallization annealing process for 50 to 100 minutes at a temperature below the γ/α transition temperature, and the second stage of the heat treatment comprising a stabilization annealing process in the temperature range of from 800° C. to 950° C. for 5 to 7 hrs, and the cooling rate during the first heat treatment stage being greater than or equal to 3° C./sec, in the temperature range between 1300° C. to 900° C.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for producing forged components of a TiAl alloy, in particular components for gas turbines, preferably aircraft turbines and in particular turbine blades for low-pressure turbines. In addition, the present invention relates to corresponding components.

Prior Art

On account of their low specific weight and their mechanical properties, components made of titanium aluminides are of interest for use in gas turbines, in particular aircraft turbines. However, in the case of titanium aluminide materials, the microstructure has to be set exactly in order to achieve the desired mechanical properties.

In this respect, there is the difficulty that special processing routes have to be chosen in order to be able to make the desired settings for the microstructure. At the same time, however, it must be possible for the production and processing of corresponding TiAl components to be carried out economically in industrial processes. Accordingly, there is a constant need to optimize microstructure settings and production routes and parameters for the production of titanium aluminide components.

Disclosure of the Invention Object of the Invention

It is therefore an object of the present invention to provide a method for producing components made of titanium aluminide materials which can be used in gas turbines, in particular aircraft turbines, preferably in the region of the low-pressure turbine, with economically reasonable production being made possible.

Technical Solution

This object is achieved by a method having the features of claim 1 and a corresponding component having the features of claim 8. Advantageous configurations are the subject matter of the dependent claims.

In the method according to the invention for producing forged components of a TiAl alloy, the components are subjected to a two-stage heat treatment after the forging, wherein the first stage of the heat treatment provides for recrystallization annealing below the γ/α transition temperature for a period of time of 50 to 100 minutes. The annealing at a temperature below the γ/α transition temperature, at which α-titanium is converted into γ-TiAl according to the phase diagram for the TiAl alloy used, can take place as close as possible to the γ/α transition temperature, where a temperature of 15° C., in particular 10° C., below the γ/α transition temperature is not to be undershot.

The recrystallization annealing can preferably be carried out for 60 to 90 minutes, in particular 70 to 80 minutes.

The first stage of the heat treatment, comprising the recrystallization annealing, is followed by a second stage of the heat treatment, comprising stabilization annealing in the temperature range of from 800° C. to 950° C. for 5 to 7 hours.

The stabilization annealing can be carried out in particular in the temperature range of from 825° C. to 925° C., preferably of from 850° C. to 900° C., with a holding period of 345 minutes to 375 minutes.

The cooling during the recrystallization annealing can be effected by air cooling, where, in the temperature range of between 1300° C. and 900° C., the cooling rate should be ≧3° C. per second, in order to set a finely lamellar microstructure of α₂-Ti₃Al and γ-TiAl in the corresponding α₂ and γ phase, which ensures the required mechanical properties. The cooling during the second heat treatment stage, i.e. the stabilization annealing, can be effected at correspondingly lower cooling rates in the furnace.

For setting the microstructure and reproducibility of a corresponding microstructure setting, it is important that the heat treatment steps are carried out as accurately as possible at the appropriately selected temperature. However, an increasingly exact setting of the temperature and retention of the components at the corresponding temperatures is associated with growing complexity, and therefore a compromise has to be found for economically viable processing. For the heat treatment of forged TiAl components, a temperature setting with an upward and downward deviation in the range of from 5° C. to 10° C. from the desired temperature has proved to be advantageous. Accordingly, the desired temperature selected for the heat treatment steps of the present invention can be set and held in a corresponding temperature window with an upward and downward deviation of 5° C. to 10° C. from the desired temperature.

Primarily titanium aluminide alloys alloyed with niobium and molybdenum can be used for the production of forged components made of TiAl alloys, in particular for gas turbine components, for example low-pressure turbine blades. Alloys of this type are also termed TNM alloys.

For the present method, use can be made of an alloy comprising 42 to 45 atom % aluminum, 3 to 5 atom % niobium and 0.5 to 1.5 atom % molybdenum.

The aluminum content can be selected to be in particular in the range of from 42.8 to 44.2 atom % aluminum, while 3.7 to 4.3 atom % niobium and 0.8 to 1.2 atom % molybdenum may be added to the alloy.

In addition, the alloy can be alloyed with boron, to be precise in the range of from 0.05 to 0.15 atom % boron, in particular 0.07 to 0.13 atom % boron.

Furthermore, the alloy can comprise unavoidable impurities or further constituents such as carbon, oxygen, nitrogen, hydrogen, chromium, silicon, iron, copper, nickel and yttrium, where the content thereof can be ≦0.05% by weight chromium, ≦0.05% by weight silicon, ≦0.08% by weight oxygen, ≦0.02% by weight carbon, ≦0.015% by weight nitrogen, ≦0.005% by weight hydrogen, ≦0.06% by weight iron, ≦0.15% by weight copper, ≦0.02% by weight nickel and ≦0.001% by weight yttrium. Further constituents can be present individually in the range of from 0 to 0.05% by weight or as a whole in the range of from 0 to 0.2% by weight.

The corresponding components can be forged by drop forging in the α-γ-β temperature range, in which case cast and/or hot-isostatically pressed blanks can be used as the primary material for the forging.

The blanks themselves can be produced by melting in vacuo or under inert gas with consumable electrodes or in a cooled crucible by means of plasma arc melting, it being possible to carry out one-off or repeated remelting of the alloy. The remelting can be effected by means of vacuum induction melting (VIM) or vacuum arc remelting (VAR), and the cast material can be subjected to hot isostatic pressing, where it is possible to employ temperatures of ≧200° C. at a pressure of ≧190 MPa and with a holding period of ≧4 hours.

According to the invention, provision is made of a component of a TiAl alloy which is produced in particular by the method presented above, in particular a component of a gas turbine, preferably of an aircraft turbine, which is made up of a triplex microstructure with a globular γ-TiAl phase, a B2-TiAl phase (body-centered cubic phase) and a lamellar α₂-Ti₃Al and γ-TiAl phase. The proportion of the γ phase here is 2 to 20% by volume, the proportion of the B2 phase is 1 to 20% by volume and the proportion of the γ phase together with the B2 phase is 5 to 25% by volume.

In particular, the proportion of the γ phase can be 5 to 15% by volume, the proportion of the B2 phase can be 3 to 15% by volume and the proportion of the two phases together can be 8 to 20% by volume.

The size of the γ phase or of the γ grains can be set such that a circumscribed circle has a diameter of ≦40 gm. The same applies to the B2 phase or B2 grains

The lamellar microstructure regions of the α₂ and γ phase have a size with which the equivalent face of a circle has a diameter of ≦100 μm.

The aspect ratio of the lamellar α₂ and γ phase, i.e. the ratio between the length and the width of the lamellae, can be ≦3:1.

The microstructure can additionally comprise borides.

EXEMPLARY EMBODIMENT

Further advantages, characteristics and features of the present invention will become clear in the following description of an exemplary embodiment.

A TiAl alloy having an aluminum content of 28.1 to 29.1% by weight, a niobium content of 8.5 to 9.6% by weight, a molybdenum content of 1.8 to 2.8% by weight, a boron content of 0.019 to 0.034% by weight, a carbon content of 0 to 0.02% by weight, an oxygen content of 0 to 0.08% by weight, a nitrogen content of 0 to 0.015% by weight, a hydrogen content of 0 to 0.005% by weight, a chromium content of 0 to 0.05% by weight, a silicon content of 0 to 0.05% by weight, an iron content of 0 to 0.06% by weight, a copper content of 0 to 0.15% by weight and a nickel content of 0 to 0.02% by weight and also an yttrium content of 0 to 0.001% by weight, remainder titanium and other individual constituents with a proportion of 0 to 0.05% by weight or together in a total of 0 to 0.20% by weight, has been melted in vacuo or under inert gas with a consumable electrode and remelted at least once in the same way. The thus melted material was subjected to hot-isostatic compaction at a temperature of >1200° C. and a pressure of more than 190 MPa for a holding period of more than 4 hours, and then forged in a drop forge at a temperature in the α-γ-β phase region. This was followed by a heat treatment comprising recrystallization annealing below the γ/α transition temperature for 75 minutes with air cooling at a cooling rate of more than 3° C. per second. Then, the corresponding component was subjected to stabilization annealing at 920° C. for 6 hours and then cooled in the furnace.

A component of this type, for example a turbine blade for a low-pressure turbine in an aircraft engine, has a triplex microstructure according to the invention with corresponding proportions of γ phase, B2 phase and lamellar α2 and γ phase. In a hot tensile test at 300° C., a component of this type has a yield strength (0.2% elongation limit R_(p0.2)) of more than 670 MPa and a tensile strength R_(m) of more than 840 MPa with a total elongation (elastic and plastic elongation to break) A_(tot) of more than 1.7%. In a hot tensile test at a temperature of 750° C., a yield strength R_(p0.2) of more than 500 MPa and a tensile strength R_(m)>730 MPa are still achieved. The creep properties are characterized by a total plastic elongation A_(p) of ≦1% at a creep temperature of 750° C. and a test stress of 150 MPa and also a creep duration of more than 200 hours.

Although the present invention has been described in detail with reference to the exemplary embodiment, it is self-evident to a person skilled in the art that the invention is not limited to this example, but rather that modifications are possible in a way that individual process and material parameters can be omitted or other combinations of process and material parameters can be chosen, without thereby departing from the scope of protection of the accompanying claims. The disclosure of the present invention encompasses in particular all individual process steps and process and material parameters. 

1.-7. (canceled)
 8. A method for producing a forged component of a TiAl alloy, wherein the method comprises forging the component and thereafter subjecting it to a two-stage heat treatment, and wherein (i) a first stage of the heat treatment comprises recrystallization annealing for 50 to 100 minutes at a temperature below a γ/α transition temperature, (ii) a second stage of the heat treatment comprises stabilization annealing in a temperature range of from 800° C. to 950° C. for 5 to 7 h, and (iii) a cooling rate during the first stage of the heat treatment is greater than or equal to 3° C./s in a temperature range of from 1300° C. to 900° C.
 9. The method of claim 8, wherein the component is a turbine blade.
 10. The method of claim 8, wherein recrystallization annealing is carried out for 60 to 90 minutes.
 11. The method of claim 8, wherein recrystallization annealing is carried out for 70 to 80 minutes.
 12. The method of claim 8, wherein stabilization annealing is carried out in a temperature range of from 825° C. to 925° C.
 13. The method of claim 8, wherein stabilization annealing is carried out in a temperature range of from 850° C. to 900° C.
 14. The method of claim 8, wherein stabilization annealing is carried out for 345 to 375 minutes.
 15. The method of claim 8, wherein a temperature during the heat treatment is set and held with an accuracy of a 5° C. to 10° C. upward or downward deviation from a desired temperature.
 16. The method of claim 8, wherein the TiAl alloy further comprises niobium and molybdenum.
 17. The method of claim 16, wherein the TiAl alloy comprises from 42 to 45 atom % of aluminum, from 3 to 5 atom % of niobium and from 0.5 to 1.5 atom % of molybdenum.
 18. The method of claim 16, wherein the TiAl alloy comprises from 42.8 to 44.2 atom % of aluminum, from 3.7 to 4.3 atom % of niobium and from 0.8 to 1.2 atom % of molybdenum.
 19. The method of claim 8, wherein the TiAl alloy further comprises from 0.05 to 0.15 atom % of boron.
 20. The method of claim 18, wherein the TiAl alloy further comprises from 0.07 to 0.13 atom % of boron.
 21. The method of claim 8, wherein the component is produced by drop forging in an α-γ-β temperature range.
 22. The method of claim 8, wherein cast or hot-isostatically pressed blanks are used as the starting materials for forging the component.
 23. A method for producing a forged component of a TiAl alloy, wherein the method comprises forging the component and thereafter subjecting it to a two-stage heat treatment, and wherein (i) a first stage of the heat treatment comprises recrystallization annealing for 70 to 80 minutes at a temperature below a γ/α transition temperature, (ii) a second stage of the heat treatment comprises stabilization annealing in a temperature range of from 850° C. to 900° C. for 345 to 375 minutes, and (iii) a cooling rate during the first stage of the heat treatment is greater than or equal to 3° C./s in a temperature range of from 1300° C. to 900° C., a temperature during the heat treatment being set and held with an accuracy of a 5° C. to 10° C. upward or downward deviation from a desired temperature.
 24. The method of claim 23, wherein the TiAl alloy comprises from 42.8 to 44.2 atom % of aluminum, from 3.7 to 4.3 atom % of niobium and from 0.8 to 1.2 atom % of molybdenum.
 25. The method of claim 24, wherein the TiAl alloy further comprises from 0.07 to 0.13 atom % of boron.
 26. The method of claim 25, wherein the component is produced by drop forging in an α-γ-β temperature range.
 27. A forged component of a TiAl alloy, wherein the component is obtainable by the method of claim
 8. 